1. Field of the Invention
The present invention relates to a radio frequency (RF) test key structure, and more particularly, to a RF test key structure that can be set in a scribe line.
2. Description of the Prior Art
Due to the rapid development of wireless communication systems, wireless devices offering great convenience are widely used in daily communications. With wireless devices, people are able to exchange information, share experiences, and communicate with each other anytime and anywhere.
Because of the wide usage of wireless devices, more various and new wireless products are developed. In wireless product fabrication, a RF device should continuously be tested during manufacturing to maintain the quality of the products. A plurality of test keys should be formed in the scribe line between two dies or on a surface of a monitor wafer. That is, as the semiconductor processes are performed, a testing device should be formed in the scribe line area utilizing the same processes at the same time for simulation. The parameters of the processes are measured by utilizing a metal probe to contact the test keys, and those parameters are important indexes used to indicate the reliability of products. As a result, a variety of defects in the semiconductor processes are monitored, and the quality of the actual device therefore is well controlled.
Please refer to FIG. 1. FIG. 1 is a schematic diagram of a traditional RF test key structure. As shown in FIG. 1, the RF test key structure 10 includes a substrate 12, a bottom metal layer and a top metal layer. The substrate 12 further includes a device under test (DUT) 18, such as a metal-oxide semiconductor (MOS). The DUT 18 includes four connecting terminals 182, 184, 186, 188. According to the figure, the four connecting terminals 182, 184, 186, 188 are a gate connecting terminal 182 in the front, a source connecting terminal 184 to the right, a drain connecting terminal 186 in the rear and a substrate connecting terminal 188 to the left respectively. The gate connecting terminal 182 is electrically connected to a gate of the DUT 18, the source connecting terminal 184 is electrically connected to a source of the DUT 18, the drain connecting terminal 186 is electrically connected to a drain of the DUT 18, and the substrate connecting terminal 188 is electrically connected to the substrate of the DUT 18. The bottom metal layer consists of a front portion 142, a right portion 144, a rear portion 146 and a left portion 148. The front portion 142, the right portion 144, the rear portion 146 and the left portion 148 surround the DUT 18 at the four edges of the DUT 18. A front signal pad 14a and a rear signal pad 14b are defined in the front portion 142 and in the rear portion 146 respectively for electrically connecting to the probes.
The top metal layer is positioned on the bottom metal layer, and a dielectric layer (not shown in the figure) is included between the top metal layer and the bottom metal layer. The top metal layer comprises a right piece 164 and a left piece 168. Utilizing a plurality of via plugs that pass through the dielectric layer, the right piece 164 and the left piece 168 are electrically connected to the right portion 144 and the left portion 148 of the bottom metal layer respectively. The right piece 164 and the left piece 168 both are narrow metal pieces parallel to a first direction 1. A ground pad 164a and a ground pad 164b are defined in the front of the right piece 164 and in the rear of the right piece 164 respectively, and a ground pad 168a and a ground pad 168b are defined in the front of the left piece 168 and in the rear of the right piece 168. As a result, the ground pad 168a, the front signal pad 14a and the ground pad 164a are arranged in one front row region, and the front row region is perpendicular to the first direction 1. The ground pad 168b, the rear signal pad 14b and the ground pad 164b are arranged in one rear row region, and the rear row region is also perpendicular to the first direction 1. Accordingly, the probes of a probe card can contact the front row region and the rear row region of the RF test key structure 10 respectively to test the DUT 18.
However, the designed structure of the traditional RF test key structure 10 is too wide to be set in a scribe line, and it increases the difficulty of manufacturing and testing. Therefore, a prior art RF test key structure positioned in a scribe line is provided. Please refer to FIG. 2. FIG. 2 is a schematic diagram of a prior art RF test key structure. As shown in FIG. 2, the RF test key structure 20 includes a substrate 22, at least four metal connecting lines 242, 244, 246, 248 and six rectangular metal pads 261, 262, 263, 264, 265, 266. The substrate 22 further includes a device under test (DUT) 28, and the DUT 28 includes a gate connecting terminal 282, a source connecting terminal 284, a drain connecting terminal 286 and a substrate connecting terminal 288 respectively. The gate connecting terminal 282 is electrically connected to a gate of the DUT 28, the source connecting terminal 284 is electrically connected to a source of the DUT 28, the drain connecting terminal 286 is electrically connected to a drain of the DUT 28, and the substrate connecting terminal 288 is electrically connected to the substrate of the DUT 28. The metal connecting lines 242, 244, 246, 248 and the rectangular metal pads 261, 262, 263, 264, 265, 266 are formed by multilevel interconnect processes. The six rectangular metal pads 261, 262, 263, 264, 265, 266 are arranged in one row, and the row is set in and parallel to a scribe line region 30. The six rectangular metal pads 261, 262, 263, 264, 265, 266 are a ground pad 261, a signal pad 262, a ground pad 263, a ground pad 264, a signal pad 265, and a ground pad 266 from left to right for electrically connecting to the probes. The metal connecting line 242 electrically connects the signal pad 262 and the gate connecting terminal 282, the metal connecting line 244 electrically connects the ground pad 266 and the source connecting terminal 284, the metal connecting line 246 electrically connects the signal pad 265 and the drain connecting terminal 286, and the metal connecting line 248 electrically connects the ground pad 261 and the substrate connecting terminal 288.
The conventional RF test key structure 20 utilizes the threadlike metal connecting line for electrically connecting. Since the electric resistance of a conductor is inversely proportional to its cross-sectional area, the threadlike metal connecting line obviously increases the inner resistance of the RF test key structure 20, and leads to a great difference between the testing data of the RF test key structure 20 and a testing data of the actual RF device. As a result, even if the RF test key structure 20 can be set in the scribe line region 30, it cannot actually reflect the electronic characteristic of the RF device, and the RF test key structure 20 therefore fails in the accuracy.